Microfabricated cantilevers find application in a range of devices. For example, microfabricated cantilevers for force sensing applications have been designed for applications such as the atomic force microscope, as explained for example in “Atomic Force Microscope”, G. Binnig, C. F. Quate and C. Gerber, Physical Review Letters, Vol. 56, No. 9, pp. 930-933 (1986).
For many purposes, micromechanical devices need to be coated by a functional metallic layer which is usually of a different composition than that of the cantilever. For instance, in force sensing applications, it is advantageous to increase the reflectivity of the cantilevers by depositing a thin gold film to improve the sensitivity of optical deflection measurements of a cantilever. In biological sensing applications, microfabricated cantilevers are often coated with a gold thin film to prior to chemical functionalization. In MEMS devices, the cantilevers are often coated with a metal to induce electrostatic actuation. In dynamic mode atomic force microscopy, cantilevers need to be actuated, Several actuation methods, especially but not exclusively those used in liquid, e.g. by laser pulses or by applying an oscillating magnetic field, also require functional coatings to enhance their effectiveness.
The deposition of a metallic thin film layer on micromechanical devices may cause a build-up of interfacial stresses due to mismatch in lattice parameters between the deposited metal and the cantilever material which is usually either silicon nitride or bulk silicon with a native oxide film. Depending on the nature of the functional coating, the compressive or tensile stresses generated by the presence of the metallic thin film may eventually be relieved by the bending of the microfabricated cantilevers. This unwanted bending of the cantilever may limit the usefulness and accuracy of subsequent measurements, and can render the cantilever unfit for purpose. In practice, the degree of bending becomes more prominent for low spring constant (<1 N/m) microfabricated cantilevers and also limits the practical thickness of the functional metallic film layer. Moreover, some functional metallic thin films such as titanium will also oxidise after their deposition on the cantilever, further exacerbating the degree of bending.
Thus, effort has been aimed at minimising the tensile or compressive stresses generated by the inclusion of the functional metallic coating on the microfabricated cantilevers or similar semiconductor based devices. Some measures to produce or maintain a straight cantilever with a functional metallic coating include applying a complex thermal annealing procedure (e.g. U.S. Pat. No. 6,103,305), balancing tensile and compressive stresses by coating the top and bottom sides of the cantilever (U.S. Pat. No. 5,866,805), applying a limited amount of coating only at the tip end of the cantilever (Atomic Force Microscopy of Local Compliance at Solid-Liquid Interfaces”, S. J. O'Shea, M. E. Welland and J. B. Pethica, Chemical Physical Letters, Vol. 223, pp. 336-340 (1994), applying low energy ion bombardment to create defects in the metallic coating as in “Relaxation of process induced surface stress in amorphous silicon carbide” by P. Argyrakis, P. McNabb, A. J. Snell, and R. Cheung, Applied Physics Letters, Vol. 89, pp. 034101 (2006) and applying complex processing parameters based on numerical models and in-process characterisation as in “Mitigation of residual film stress deformation in multilayer microelectromechanical systems cantilever devices” by J. S. Pulskamp, A. Wickenden, R. Polcawich, B. Piekarski, and M. Dubey, Journal of Vacuum Science and Technology B, Vol. 21, pp. 2482-2486, (2003). However, none of these methods above can be readily implemented in the fabrication process, resulting in high production costs and/or limitations to the types of functional metallic coatings.
Accordingly, it would be desirable to provide a microfabricated cantilever with a functional metallic coating and method for fabricating the same without unwanted bending of said cantilever.